What is the difference between ATM and EFM?

Hello,
both are interfaces for transmission over gshdsl/gshdslbis links, there are others like G703 which can also be transported this way over a g.shdsl link.
ATM (Asynchronous Transfer Mode) is the older version adapted to old DSLAMs that only support ATM.
An additional ATM encapsulation (fixed cells of 53 bytes + AAL5) makes it less efficient than the more modern EFM.
On the router CPE and DSLAM in ATM, one or more ATM PVCs are configured identified by their VPI/VCI on one or more ATM sub-interfaces.
When configuring the shdsl controller in ATM, the ATM interface is created automatically.
EFM (Ethernet in the First Mile) is the modern version currently under significant development and uses a different encapsulation in shdsl to transmit the equivalent of Ethernet frames, thus variable size from 64 to... more than 1500 bytes and with less encapsulation.
EFM can also offer multiple links on the same connection using traditional Ethernet VLANs.
On a CPE router, when configuring in EFM mode, a virtual Ethernet interface is created automatically.
Both provide speeds up to 21Mbit/s (DSL speed) over 4 pairs and g.shdslbis.
The main advantage of ATM mode is the high capacity for QoS adjustment on ATM switches.
The main advantage of EFM is less encapsulation and greater resilience:
a multipair EFM link continues to operate at reduced speed, of course, if one pair is missing, while in ATM, the entire link is cut off if one telephone pair is defective.
Both are generally used for internet access with a PPP overlay above, allowing more flexibility (PPPOA or PPPOEOA for ATM and PPPOE for EFM) represented by a dialer interface on a CPE.
With the generalization of Ethernet DSLAMs instead of ATM DSLAMs, it is likely that EFM will replace ATM on symmetrical access links in the local loop.
and ... There you go!

Thank you very much! Brupala